Affecting the game as gragas mid.

I like playing full ap gragas mid and have made it to plat 4. A common occurrence is i do pretty well early game and have some kills but then go to a side lane and cant seem to kill the laner or cant get to my team to help. I do usually run ignite. In team fights engaging can be super tough without a good ult knockback. If i body slam at the wrong time, I am just stunned and dead. Zhonyas is an expensive item to try to get before my core items. How do I go about roaming early game when I am fed without losing too much farm or my turret? Top lane is another challenge all together so I just stay mid. Any tips would be awesome. Thank you.